The invention relates to a method of manufacturing an electric lamp having a glass tubular part with a longitudinal axis, which part is sealed by a sealing member. A constriction with a collar is formed by means of heating at one end of the tubular part in which subsequently the sealing member is secured in a gas-tight manner. The invention also relates to an electric lamp obtained by means of such a method and to a device for performing such a method.
Such a method is generally known, particularly in the manufacture of tubular low-pressure mercury vapour discharge lamps. For securing a sealing member such as a "stem", a constriction is provided in the end of a discharge tube by heating the end of the glass wall at its end until the glass softens and by subsequently giving the glass wall of the end the desired shape by means of a profiling tool so that a collar is obtained. The discharge tube is sealed in a gas-tight manner and is subsequently exhausted by means of the exhaust tube present in the stem as described, for example, in British Patent Specification No. 1,475,458. A stem is herein understood to mean a glass portion comprising an exhaust tube and electrode pins between which an electrode is arranged.
In the known method, a tube is cut to the desired length in a first fixture whereafter the constriction with the collar is formed in a second fixture. The use of two differnt fixtures easily leads to inaccuracies in the shape and dimensions of the end of the tubular part. This is notably unwanted in a bulk manufacturing process because then a reliable sealing by means of the sealing member is obtained to an insufficient extent.